1. Field of the Invention
This invention relates to a process for producing methacrylic resin sheets and, more particularly, to a process for more efficiently producing methacrylic sheets with increased productivity achieved by using specific combinations of polymerization initiators and process conditions for polymerizing the methacrylic monomer either alone or in combination with another acrylic monomer.
2. Description of the Prior Art
In the production of polymethyl methacrylate sheets by cast polymerization, a large number of polymerization initiators have heretofore been proposed in attempts to improve productivity. For example, polymerization initiators having high activity at low temperatures, such as those having a 10-hour half-life temperature of 80.degree. C. or below, preferably 60.degree. C. or below, have been used because they can promote the polymerization.
However, the use of such polymerization initiators having a low half-life temperature increases the amount of unreacted monomer remaining after completion of the polymerization step and accordingly, degrades the quality of the resulting cast sheets. In order to decrease the amount of unreacted monomer, composite polymerization initiators have been proposed such as those comprising a combination of a low-temperature polymerization initiator and a hightemperature active polymerization initiator including those described in Japanese Patent Laid-Open Nos.33993/'74, 725/'80 and 145907/'81, and Japanese Patent Application Publication No. 16762/'90.
However, the processes disclosed in Japanese Patent Application Laid-Open Nos.33993/'74 and 725/'80 do not necessarily employ polymerization conditions which bring about a satisfactory improvement in productivity.
On the other hand, the process disclosed in Japanese Patent Application Laid-Open No 145907/'81 is considerably more effective in improving productivity, depending upon the specific combination of polymerization initiators actually used. However, from the viewpoint of utility, some difficulties are encountered in using polymerization initiators having a 10-hour half-life temperature of 50.degree. C. or below. Additionally, in this publication there is no disclosure given or any detailed explanation provided of the actual polymerization conditions used.